1. Field of the Invention
The present invention relates to an organic electroluminescence device, and more particularly, to red phosphorescence compounds and organic electroluminescence device using the same. Most particularly, the present invention relates to red phosphorescence being used as a dopant of a light emitting layer of an organic electroluminescence device, which is formed by serially depositing an anode, a hole injecting layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injecting layer, and a cathode.
2. Discussion of the Related Art
Recently, as the size of display devices is becoming larger, the request for flat display devices that occupy lesser space is becoming more in demand. Such flat display devices include organic electroluminescence devices, which are also referred to as an organic light emitting diode (OLED). Technology of such organic electroluminescence devices is being developed at a vast rate and various prototypes have already been disclosed.
The organic electroluminescence device emits light when electric charge is injected into an organic layer, which is formed between an electron injecting electrode (cathode) and a hole injecting electrode (anode). More specifically, light is emitted when an electron and a hole form a pair and the newly created electron-hole pair decays. The organic electroluminescence device can be formed on a flexible transparent substrate such as plastic. The organic electro-luminescence device can also be driven under a voltage lower than the voltage required in a plasma display panel or an inorganic electroluminescence (EL) display (i.e., a voltage lower than or equal to 10V). The organic electroluminescence device is advantageous in that it consumes less energy as compared to other display devices and that it provides excellent color representation. Moreover, since the organic EL device can reproduce pictures by using three colors (i.e., green, blue, and red), the organic EL device is widely acknowledged as a next generation color display device that can reproduce vivid images.
The process of fabricating such organic electroluminescence (EL) device will be described as follows:
(1) An anode material is coated over a transparent substrate. Generally, indium tin oxide (ITO) is used as the anode material.
(2) A hole injecting layer (HIL) is deposited on the anode material. The hole injecting layer is formed of a copper phthalocyanine (CuPc) layer having a thickness of 10 nanometers (nm) to 30 nanometers (nm).
(3) A hole transport layer (HTL) is then deposited. The hole transport layer is mostly formed of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB), which is treated with vacuum evaporation and then coated to have a thickness of 30 nanometers (nm) to 60 nanometers (nm).
(4) Thereafter, an organic light emitting layer is formed. At this point, a dopant may be added if required. In case of green emission, the organic light emitting layer is generally formed of tris(8-hydroxy-quinolate)aluminum (Alq3) which is vacuum evaporated to have a thickness of 30 nanometers (nm) to 60 nanometers (nm). And, MQD(N-Methylquinacridone) is used as the dopant (or impurity).
(5) Either an electron transport layer (ETL) and an electron injecting layer (EIL) are sequentially formed on the organic emitting layer, or an electron injecting/transport layer is formed on the organic light emitting layer. In case of green emission, the Alq3 of step (4) has excellent electron transport ability. Therefore, the electron injecting and transport layers are not necessarily required.
(6) Finally, a layer cathode is coated, and a protective layer is coated over the entire structure.
A light emitting device emitting (or representing) the colors of blue, green, and red, respectively, is decided in accordance with the method of forming the light emitting layer in the above-described structure. As the light emitting material, an exciton is formed by a recombination of an electron and a hole, which are injected from each of the electrodes. A singlet exciton emits fluorescent light, and a triplet exciton emits phosphorescence light. The singlet exciton emitting fluorescent light has a 25% probability of formation, whereas the triplet exciton emitting phosphorescence light has a 75% probability of formation. Therefore, the triplet exciton provides greater light emitting efficiency as compared to the singlet exciton. Among such phosphorescence materials, red phosphorescence material may have greater light emitting efficiency than fluorescent materials. And so, the red phosphorescene material is being researched and studied as an important factor for enhancing the efficiency of the organic electroluminescence device.
When using such phosphorescence materials, high light emitting efficiency, high color purity, and extended durability are required. Most particularly, when using red phosphorescence materials, the visibility decreases as the color purity increases (i.e., the X value of the CIE chromacity coordinates becomes larger), thereby causing difficulty in providing high light emitting efficiency. Accordingly, red phosphorescence material that can provide characteristics of excellent chromacity coordinates (CIE color purity of X=0.65 or more), enhanced light emitting efficiency, and extended durability needs to be developed.